High-temperature sintering is essential for the densification of garnet-type solid electrolytes to achieve high ion conductivity and suppression of Li-dendrite growth. In this study, the densification and electrochemical performance of Li6.4La3Zr1.4Ta0.6O12 (LLZT) electrolyte was optimized by controlling the oxygen content of the atmosphere during sintering. Electrochemical tests were performed on sintered and densified pellets at oxygen contents between 0.005 and 99.995 vol.% using a gassing approach. The relative density and critical current density were optimized to 97.6% and 1.0 mA/cm2, respectively, at a high oxygen content. As oxygen content increased, sintered pellets exhibited increased lithium-oxide content by inhibiting Li-loss, facilitating densification. These results are expected to promote the implementation of high-quality LLZT electrolyte and solid-state batteries by exploiting the role of oxygen content during sintering. This study shows that high-quality manufacturing can be achieved with low-cost, pressureless sintering.